THE SKULLS OF AMrHIBIA. 213 



— the absence of branchiostegal rays, and the presence of two 

 "jugular' plates between the mandibular rami in Pohjpterus, 

 must not be overlooked even in tliis brief enumeration of a few 

 of the most salient modifications of the skulls of osseous fishes. 



THE SKULLS OF AMPHIBIA. 



In cranial structure, as in all the other more important 

 features of their organization, the Amphibia are closely allied to 

 Fishes, and widely separated from the abranchiate Vertebrata. 



As in Fishes, a single median membrane bone, or parasphe- 

 noidj is developed under the base of the skull, while no such 

 median bone is found in the higher Vertebrata. Like Lepido- 

 siren, the Amphibia have no ossified basi-occipital or supra- 

 occipital, whereas all the abranchiate Vertebrata possess these 

 bones. 



Again, like Lepidosiren and many other Fishes, the Amphibia 

 have no true basi-sphenoid, developed in the cartilage of the 

 basis cranii ; while all the abranchiate Vertebrata have that bone 

 well developed. 



The hyoidean apparatus is, in Amphibia, as in Fishes,* con- 

 nected with a suspensorium common to it and the mandibular 

 apparatus. In all the higher Vertebrata the hyoidean apparatus, 

 if it is attached directly to the skull at all, is united therewith 

 separately and distinctly. 



In all Amphibia which have ossified ex-occipitals, a condyle 

 is developed on each, for articulation with the first vertebra of the 

 spinal column ; and the basi-occipital, remaining unossified, 

 takes no share in the formation of these condyles. In all the 

 higher Vertebrata, on the other hand, the bony basi-occipital 

 takes a greater or less share in the formation of the occipital 

 condyle, or condyles. 



The skull of Amphibia resembles that of the Chimseroids and 

 Lejnclosiren, and differs from that of Teleostean, Ganoid, and 

 Plagiostome fishes, in the absence of any natural division between 

 the palato-quadrate and suspensorial cartilages. 



* According to Stannius, however, the hyoidean arch is attached directly and 

 independently to the skull in many Eays. See that author's admirable " Handbuch 

 der Anatomie der Wirbelthiere," Erster Buch, p. 46. 



